1. Field of the Invention
The present invention relates to a light-emitting device using a group III nitride compound semiconductor and a method for manufacturing such a device. In particular, the present invention relates to the structure of a mirror structure formed on the reverse side of a substrate and a method for forming the mirror structure.
2. Description of the Related Art
A variety of light-emitting devices using group III nitride compound semiconductor comprising a mirror structure formed on the reverse side or backside of a substrate and a variety of methods for manufacturing such devices are generally known in the art. For example, Japanese patent Application Laid-Open (kokai) No. 11-126924 (Title: “Method of manufacturing gallium nitride compound semiconductor element”) (hereinafter referred to as Reference 1), Japanese Patent Application Laid-Open No. 11-126925 (hereinafter referred to as Reference 2), Japanese Patent Application Laid-Open No. 5-129658 (hereinafter referred to as Reference 3), and Japanese Patent Application Laid-Open No. 11-261112 (hereinafter referred to as Reference 4) disclose other such LEDs.
FIG. 14 shows a cross-sectional view of a conventional light-emitting semiconductor device 900 using a group III nitride compound semiconductor of the type disclosed in the aforementioned Reference 1.
A sapphire substrate 11 is formed into an approximately square shape. A buffer layer 12 and an n-type contact layer 13 (hereinafter alternatively referred to as “a high carrier concentration n+-layer 13” or “an n+-layer 13”) are then sequentially formed on the substrate 11. An n-type clad layer 14 is then formed on the high carrier concentration n+-layer 13.
An emission layer 15 having a multiple quantum well (MQW) structure comprising a plurality of alternating barrier layers 151 and well layers 152 is then formed on the n-type clad layer 14. A p-type clad layer 16 is then formed on the emission layer 15 and a p-type contact layer 17 is formed on the p-type clad layer 16.
A positive electrode 18A which transmits light is formed on the p-type contact layer 17. The positive electrode 18A comprises a first thin-film metal layer which is adjacent to the contact layer 17 and a second thin-film metal layer which is adjacent to the first thin-film metal layer. A negative electrode 18B, which is formed on the n+-layer 13, comprises a plurality of metal film each film having a multiple layer structure. A metal electrode pad 20 is then formed on a portion of the positive electrode 18A.
On the reverse side of the substrate 11 (sapphire substrate 11), a metal layer 90 (mirror structure 90) consisting of about 200 nm of aluminum (Al) is then formed.
Generally, a sapphire substrate is hard. In order to divide a semiconductor wafer precisely, therefore, separation grooves or split lines (also referred to as scribe lines) are formed on both the semiconductor side and the reverse side of the substrate after forming the electrodes.
When split lines are formed on the electrode side (upper side) of the semiconductor wafer, the substrate is still required to maintain a certain mechanical strength. A certain thickness of the substrate is preferred, typically about several hundred microns (μm). Then, in order to separate the individual light-emitting semiconductor devices properly, the substrate is polished to further reduce its thickness. The split lines are then scribed on the reverse side, or the polished surface, of the substrate.
In the conventional manufacturing processes, however, leave two significant problems unsolved. As a result, the mass production, sale, and use of such light-emitting devices using group III nitride compound semiconductor and including a reflective metal layer has not been easy.
The first problem relates to the formation of the split lines.
Before forming the metal layer on the reverse side of the substrate, it is necessary to form split lines (scribe lines) on the reverse side of the semiconductor wafer or substrate.
This is because scribing cutters are typically formed from small grains of diamond shaped into a blade. Because the blade of such a scribing cutter will tend to be clogged by any metal layer laminated on the reverse side of the substrate, the split lines cannot be formed after the metal layer is formed.
Further, the scribing cutter must be positioned precisely to cut only the predetermined regions. However, a metal layer formed on the surface to be cut would hinder the positioning. To solve this problem, the conventional method, i.e., not forming the metal layer on predetermined regions of the reverse side of the substrate used for positioning, as disclosed in the above-mentioned references, may be used. By using such a conventional method, however, semiconductor light-emitting devices are formed without the necessary metal layer to form the mirror structure. Because these devices cannot be commercialized, a portion of the wafer is wasted and production efficiency is reduced.
Yet another problem associated with the conventional scribing process is that a semiconductor wafer being scribed is typically fixed to an adhesive sheet. When the metal layer is being formed on the reverse side of the substrate, a portion of the sheet materials, in particular the adhesive, tends to volatilize and release abundant and undesirable gases during the deposition of the metal layer. Because these gases interact chemically with the deposited or sputtered metals, the reflectivity and affinity (adhesion) of the resulting metal layer to the substrate can be significantly lowered.
Another significant problem relates to the corrosion resistance of the resulting mirror structure.
When adhesives (such as paste materials including silver are used to bond the mirror structure of the light-emitting device to other structures such as leadframes, submounts, and stem contacts, the resulting alloying and oxidation effects can cause the metal layer to deteriorate and, as a result, lower the reflectivity of the mirror structure.
The reflective layer of the light-emitting device may also be damaged during and/or after the scribing and separation processes. When the reflection layer is too severely damaged, quantity of reflected light will decrease and the reflective layer may be partially or completely peeled off (delamination).